CARACAS, Jan 15 (Reuters) - Venezuelan Vice President Nicolas Maduro gave a brief state of the nation address on Tuesday in place of his ailing boss, Hugo Chavez, who has not been seen in public since going to Cuba for cancer surgery more than a month ago.

Maduro stood in for Chavez with a 10-minute speech to Congress in which he defended the president's decision to rule the OPEC nation from a hospital bed in Havana, despite opposition calls for him to step aside and name a temporary leader.

"We are following the constitution in an impeccable manner," Maduro, Chavez's heir apparent, told lawmakers, holding up a copy of the 1999 charter Chavez helped write.

The brevity of the speech, in sharp contrast to Chavez's nine-and-a-half-hour address last year, came as a surprise following Maduro's month-long effort to impersonate Chavez's bombastic charisma.

Maduro offered no new information about the president's cancer, which threatens to upend Chavez's self-styled socialist revolution and convulse the political order of a country that holds the world's biggest oil reserves.

Maduro said Chavez had named a former vice president, Elias Jaua, as the new foreign minister, a move that supporters will likely point to as a sign that the president is in control of governance despite his prolonged absence.

Chavez said last year he was completely cured of cancer and told parliament he believed God had sent him the disease to help him "see better, think better, and study better." He went on to win a new six-year term at an election in October.

Within weeks of his victory, however, the 58-year-old had to return to Cuba for more treatment. He underwent his fourth cancer operation in 18 months on Dec. 11, and has suffered multiple complications since then.

OPPOSITION OUTRAGED

Opposition leaders pounced on Maduro's address as a further sign of institutional decay caused by the leader of a sovereign nation governing in absentia from overseas.

"We are facing an illegitimate government," said opposition stalwart Maria Corina Machado. "We demand that decisions about Venezuela be made in Venezuela."

Should Chavez step down or die, a new election would likely pit Maduro, 50, against opposition leader Henrique Capriles, 40, the governor of Miranda state who lost to Chavez in October.

"The only way a minister can be named is through a decree by the president of the republic," Capriles said in a statement.

Jaua was tipped as a possible ambassador to Argentina in 2002, but rejected by the government there because of his radical past.

In an update on Chavez's health on Sunday, the government said a lung infection had been controlled and that his condition was improving. It said he still needed help breathing, but was conscious. Chavez has never said what type of cancer he has.

Maduro said earlier on Tuesday that senior members of the government visiting Havana had briefed Chavez on Monday.

"He asked (Oil Minister) Rafael Ramirez about things ... we could say our commander is climbing the hill again, he is advancing," Maduro told a meeting of state governors at which he shook hands with Capriles.

The opposition called on the Washington-based Organization of American States this week to give it the chance to speak at a session about Chavez's absence and what it said were developments that threatened democracy in Venezuela.

'EXTREME RIGHT'

Ramirez dismissed those allegations, saying a group of 14 opposition lawmakers who walked out of Maduro's speech had also played an active role in a brief coup against Chavez in 2002.

"They are the most extreme of the Venezuelan right," the oil minister told reporters at parliament. "They're left isolated."

Opposition leaders say a caretaker president should be appointed and new elections held within 30 days of Chavez's absence being made formal, as called for in the constitution.

Chavez's planned inauguration on Jan. 11 was postponed, but the government says he remains president and that the ceremony can be performed later before the Supreme Court.

Raising the risk of a confrontation, both the opposition and the ruling Socialist Party (PSUV) are planning marches in the capital next Wednesday, Jan. 23.

The date is an emotive one for Venezuelans, recalling the day in 1958 when military dictator Marcos Perez Jimenez fled the country amid widespread rioting and a coup by rebel soldiers.

Capriles has said any violence would only benefit the "chavista" authorities ruling Venezuela in the president's absence. Both sides have urged their followers not to fall for provocations.

In the meantime, economic policy decisions appear to be on hold, including an expected devaluation of the bolivar currency that economists and the private sector say is long overdue.

The main business chamber, Fedecamaras, released a statement on Tuesday bemoaning the pressing need to deal with growing imbalances in the economy of South America's top oil exporter.

"We are in an urgent moment that needs, without delay, the adoption of rational and sound economic decisions," it said.

"Insecurity, instability, uncertainty and misguided economic policies are the real cause of shortages and inflation."

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